The Expanding Universe: What Does "Creating Space" Really Mean?

The Paradox of Space Creation

Hubble's law and the measurement of expansion

In 1929, Edwin Hubble provided the first observational evidence of the expansion of the Universe by establishing the relation between the distance of galaxies and their recession velocity. This relation, known as Hubble's law, is expressed by the formula v = H₀ × d, where v is the recession velocity, d the distance and H₀ the Hubble constant.

Cosmic acceleration and dark energy

In 1998, two independent teams studying type Ia supernovae made a revolutionary discovery: the expansion of the Universe is not slowing down as expected, but is accelerating. This observation, awarded the 2011 Nobel Prize in Physics, implies the existence of a repulsive energy, dubbed "dark energy", which would make up about 68% of the energy content of the Universe.

The table below summarizes the composition of the Universe and the role of each component in the dynamics of expansion.

Does the Expanding Universe "Create Space"?

The concept of comoving points

The concept of "space creation" in the expanding Universe is subtle and often misinterpreted. It is not about the appearance of new space from "nothing" in the philosophical sense, but about the increase of the proper distance between comoving points of spacetime.

N.B.:
Comoving points designate fixed coordinates in the expanding Universe. A distant galaxy retains its comoving position, even though the distance separating us from it increases with the scale factor. In other words, it does not "move" locally but follows the expansion of the cosmological metric.

If it is not space, what is "created"?

The dynamic geometry of the Universe

When we say that the Universe "creates space", we must not imagine a manufacture of vacuum, as if the Universe were producing some kind of invisible matter. In reality, what evolves is the geometry of the Universe, that is, the way distances are measured between two points.

"Expansion" concerns the way we measure distances

In physicists' language, the very structure of spacetime is dynamic: the coordinates of distant galaxies remain fixed (they are said to be comoving), but the "ruler" used to measure distances changes over time. It is therefore not a material "fabric" that dilates, but the metric that evolves.

In other words, the Universe does not add volume as one inflates a balloon with air, it recomputes continuously what it means to be separated by a million light-years. What is "created" is neither vacuum nor a mysterious substance, but a relative increase of distances between distant cosmic objects.

A mathematical concept rather than a physical creation of space

Saying that the Universe "creates space" is an imagery way of translating a mathematical reality. In cosmology, one does not manipulate a material fabric but a metric, that is, a formula that indicates how to calculate distances between two points of the Universe. When accelerated expansion is at play, it is this metric that changes with time.

Thus, it is not "space" as a substance that multiplies, but the geometric relation between points that transforms. The Friedmann-Lemaître-Robertson-Walker (FLRW) metric provides the framework: it shows that the scale factor increases, which is equivalent to saying that two comoving galaxies see their distance grow even though they do not move locally.

This vocabulary is useful for popularization, but one must keep in mind that it is a shortcut: what is really described is a dynamic evolution of the geometric equations of general relativity: \( G_{\mu\nu} + \Lambda g_{\mu\nu} = \frac{8 \pi G}{c^4} T_{\mu\nu} \)

Galaxies that "move away"?

An apparent recession, not a local motion

In many illustrations, galaxies are drawn ever farther apart, as if they were really moving through an already existing space. But within the framework of general relativity, that is not how expansion should be understood. Distant galaxies do not "travel" across the cosmos, they keep their position in the comoving grid, while it is this grid, the metric itself, that is measured differently over time. In other words, it is the metric itself that changes scale with time. It is not a substance that deforms, but the rules of measurement (distances, intervals) that stretch.

N.B.:
Very distant galaxies receding at so-called "faster-than-light" speeds do not violate special relativity, because they do not correspond to a local motion but to the evolution of geometry itself.